In general, a compression refrigerating machine is constituted at least by a compressor, a condenser, an expansion mechanism (such as an expansion valve), an evaporator and optionally a dryer, and has a structure where a mixed liquid of a refrigerant and a lubricating oil (i.e., a refrigerating machine oil) is circulated in a closed system. In the compression refrigerating machine, in general, a high temperature occurs in the compressor, whereas a low temperature occurs in the condenser while depending on the type of the machine, and it is therefore necessary that the refrigerant and the lubricating oil are circulated in the system without phase separation over the wide temperature range of from the low temperature to the high temperature. The refrigerant and the lubricating oil generally have regions where phase separation occurs on the low temperature side and the high temperature side. The maximum temperature of the phase separation region on the low temperature side is preferably −10° C. or less, and particularly preferably −20° C. or less, and the minimum temperature of the phase separation region on the high temperature side is preferably 30° C. or more, and particularly preferably 40° C. or more. The phase separation that occurs during operation of a refrigerating machine exerts considerable adverse affects on the service life and the efficiency of the machine. For example, the phase separation of the refrigerant and the lubricating oil that occurs in the compressor portion causes lubrication failure in the moving parts therein, which considerably shortens the service life of the machine due to occurrence of seizure or the like, and the phase separation that occurs in the evaporator portion brings about decrease of the heat exchange efficiency due to the presence of the lubricating oil having high viscosity.
As a refrigerant for a refrigerating machine, a chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC) and the like have been mainly used, but these are compounds containing chlorine, which causes the environmental issue (ozone layer destruction), and therefore an alternative refrigerant containing no chlorine, such as hydrofluorocarbon (HFC), is being studied. As the hydrofluorocarbon, for example, a hydrofluorocarbon, which is represented by 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane and 1,1,1-trifluoroethane (which are hereinafter referred to as R134a, R32, R125 and R143a, respectively), is receiving attention, and R134a is used, for example, in a car air conditioner system.
In view of potential issue on the global warming of HFC, however, a so-called natural refrigerant, such as carbon dioxide, receives attention as an alternative refrigerant suitable for environmental protection, but carbon dioxide requires a high pressure and thus cannot be used in an existing car air conditioner system.
As a refrigerant that has a low global warming potential and is capable of being used in an existing car air conditioner system, refrigerants that have a particular polar structure in the molecule thereof have been found, for example, the unsaturated fluorinated hydrocarbon compound disclosed in Patent Document 1, and the fluorinated ether compound, the fluorinated alcohol compound and the fluorinated ketone compound disclosed in Patent Document 2.
Furthermore, Patent Documents 3, 4 and 5 disclose the techniques of including the terpene compound or the ester compound of the particular fatty acid and polyhydric alcohol in a base oil for the purpose of achieving the thermal and chemical stability and the refrigerant compatibility at high levels even upon using with the aforementioned refrigerants.
A lubricating oil for a refrigerating machine that is used with those refrigerants is thus demanded to have excellent compatibility with the refrigerants and to be excellent in stability.